M4 makes a good exhaust system and it'll give you a little more power...and don't let anyone tell you there isn't quite a bit more hiding inside the engine cases...because there is! However..there's not much 'low hanging fruit' to pick so to speak, so if you want more power you're going to have to work a bit for it. The Sprint air filter element is a nice addition that frees up a bit of power. The stock K3-4 intake snorkel is really too long and they made it shorter in K5+ bikes to make better power. Be sure there's a snorkel in the airbox! Taking it out only makes more noise and actually hurts the torque production. Make sure the engine has fresh plugs and is tuned properly...including valve adjustment. Remove the STV plates...they're just restrictions and do nothing to help you. The K3-4 engines have a very heavy alternator rotor...K5+ have a lighter one by about 2 lbs which makes for a much snappier motor.
Most 'lack of power' complaints come down to fuel system problems....clogging inlet screen or HP filter. Beyond these simple things you're now looking at adjusting the cam phasing, cutting material off of the top surface of the jugs to raise compression while decreasing the squish clearance. Set the cams at 106/109 to start and with the M4 (and the rest of the other things) you'll have the majority of the potential to be had.
